In response to security threats in transportation and public spaces, inspection of people for materials such as narcotics and explosives and other types of contraband is becoming common at security checkpoints at, for example, airports, train stations, sporting events, concerts, federal buildings, and other public and private facilities. Systems using non-ionizing radiation, such as terahertz and millimeter range radiation, for imaging of concealed items of interest are known. Such systems are described in, for example, WO200875948, U.S. Pat. No. 7,304,306, and U.S. Pat. No. 7,295,019.
WO200875948 describes analysis of imaging radiation to visualize properties of the human internal body, such as electrical conductivity, electric permittivity, and magnetic permeability. Non-coherent waves having a large bandwidth are transmitted, to ensure that they are harmless. The bandwidth is preferably half of a control frequency. The radiation source may for example be a broadband non-coherent noise generator.
U.S. Pat. No. 7,304,306 describes an imaging system using terahertz radiation, and a direct conversion detector module is used. Detected radiation is split into several components, which are attenuated. The objective is to detect explosive material. The different signals are combined to provide a composite signal.
U.S. Pat. No. 7,295,019 describes use of capacitive and inductance sensors for detecting concealed items such as plastic explosives and ceramic knives.
The invention is directed towards providing for improved detection of non-metallic concealed items, particularly with improved differentiation between threat and non-threat items.